516 Forbes — The Microscope in Geology. 



2. Bocks built up of the more or less rounded or angular debris of 

 previously existing sedimentary or eruptive rocks. — Where sufficiently 

 coarse-grained, these rocks constitute ordinary conglomerates, brec- 

 cias, grits, sandstones, etc., and are easily analysed by the eye ; but 

 if fine, as shales, slates, etc., the microscope must be appealed to, in 

 order to resolve them into their constituent mineral or rock particles, 

 and by this means it will be seen that even the most compact and 

 homogeneous specimens are a mere aggregate of more or less rounded 

 and vp-ater-worn grains of quartz, weathered felspar, mica, chlorite, 

 soft and hard clays, clay slate, oxide of iron, iron pyrites, carbonate 

 of lime, fragments of fossil organisms, etc., arranged without any 

 trace of decided structure or crystallisation, even when the highest 

 powers of the microscope are employed in their examination. The 

 physical structure and optical properties of the mineral components 

 enable them, however, to be recognised with great certainty, even 

 "when in grains of less than ~ of an inch in diameter. 



A section is given of a fine-grained (uncleaved) Silurian clay slate 

 from Sorata, in Bolivia, magnified 400 linear. This rock is com- 

 posed of irregular grains of quartz sand, weathered felspar, and 

 water- worn mica, along with specks of oxide of iron and iron pyrites, 

 all promiscuously mixed. 



In the case of roofing slate, however, the microscope shows that 

 the constituents, instead of being distributed at random throughout 

 the mass, possess a definite arrangement, as may be seen in a 

 section of Lower Silurian roofing slate from the Festiniog quarries, 

 where they are disposed in parallel lines, thus constituting lines of 

 weakness or the cleavage of the slate. The researches of Sharpe and 

 Sorby have conclusively proved that this has not resulted from any 

 chemical or crystalline action whatsoever, the particles being in 

 themselves perfectly unaltered; and that the arrangement is solely due 

 to the effects of pressure, applied at right angles to the structure 

 itself, thereby causing an elongation or flattening out of some, along 

 with a sliding movement of other of the particles. The amount of 

 compression to which an ordinary roofing-slate has been subjected 

 in one direction, has been calculated, approximating from the elong- 

 ation or distortion of the particles, to be about equal to one-half of 

 its original volume. 



Besides the cleavage structure, so produced by the compression of 

 rocks whilst in a more or less plastic state, Mr. Sorby has shown that 

 another system of minute jointing may also be present in these rocks, 

 the serrated edges of which, as seen by the microscope, prove it to 

 have been the result of force applied to the rock subsequently to its 

 having been in a perfectly rigid condition. 



Eocks of this class, when somewhat close-grained and much in- 

 durated, have not unfrequently, from their external appearance, been 

 mistaken for intrusive rocks : thus an Upper Oolitic, highly -inclined 

 shale-bed, was mapped by D'Orbigny as an eruptive greenstone ; but 

 the microscopic structure proves the contrary most conclusively. 



3. Hocks composed of mineral substances extracted from aqueous solu- 

 tion hy crystallisation, jarecipitation, or the action of organic life. — 



